The present invention relates to a modem having echo cancellers, and more particularly to a modem for two-wire full-duplex communications having two echo cancellers for a near end echo and a far end echo.
A modem for two-wire full-duplex communications (hereinafter referred to simply as "modem") has to cancel a near end echo originating from a hybrid transformer of a local modem and a far end echo which results from the return to the local modem of an echo generated at a hybrid transformer of a remote modem in response to data sent from the local modem.
Since a change in the opposite party participating in a communication usually entails a variation in communication routes or paths, the round-trip delay from the time a datum is transmitted until it comes back from this far end as an echo also varies correspondingly. A prior art modem copes by means of such variations in round-trip delay with a variable delay circuit for delaying the start timing of far end echo cancellation. The modem is arranged before a far echo canceller. This far end echo, when passing a converter on the transmission path, undergoes a phase roll due to the frequency offsetting or phase jitter of the carrier. Since the phase roll increases the uncancelled residue of the far end echo, the far echo canceller is provided with a phase corrector for automatic adaptive correction of the phase roll.
In the prior art modem, if the round-trip delay is short, the time regions of the near end echo cancellation and of the far end echo cancellation for one transmitted datum will overlap each other. In such an overlapped time region, two echo cancellers, one for the near end echo and the other for the far end echo, cancel the echoes of the same transmitted datum at the same time. As a result, the convergence velocity in training is significantly reduced, or even divergence occurs instead of convergence, to obstruct the start of communications. This problem is especially serious where the far end echo canceller has a phase corrector.